Mosaic Nanocrystalline Graphene Skin Empowers Highly Reversible Zn Metal Anodes

Abstract Constructing a conductive carbon‐based artificial interphase layer (AIL) to inhibit dendritic formation and side reaction plays a pivotal role in achieving longevous Zn anodes. Distinct from the previously reported carbonaceous overlayers with singular dopants and thick foreign coatings, a new type of N/O co‐doped carbon skin with ultrathin feature (i.e., 20 nm thickness) is developed via the direct chemical vapor deposition growth over Zn foil. Throughout fine‐tuning the growth conditions, mosaic nanocrystalline graphene can be obtained, which is proven crucial to enable the orientational deposition along Zn (002), thereby inducing a planar Zn texture. Moreover, the abundant heteroatoms help reduce the solvation energy and accelerate the reaction kinetics. As a result, dendrite growth, hydrogen evolution, and side reactions are concurrently mitigated. Symmetric cell harvests durable electrochemical cycling of 3040 h at 1.0 mA cm−2/1.0 mAh cm−2 and 136 h at 30.0 mA cm−2/30.0 mAh cm−2. Assembled full battery further realizes elongated lifespans under stringent conditions of fast charging, bending operation, and low N/P ratio. This strategy opens up a new avenue for the in situ construction of conductive AIL toward pragmatic Zn anode.

Three types of CR2032 coin cells were assembled to investigate the electrochemical performances, including Zn-Zn symmetric cell, Ti-Zn asymmetric cell, and Zn-KVOH full cell. As-prepared electrodes were cut into circle discs. The cells were assembled with the commercial glass fiber as separator (Whatman TM ) and 2 M ZnSO 4 as electrolyte. For the flexible full cells, the cathode, anode and glass fiber separator were all cut into rectangles, with the size of 3 cm × 2 cm, 3 cm × 2 cm, and 4 cm × 3 cm respectively. The KVOH cathode, separator and NOC@Zn anode were sealed by polyimide tape. Then the flexible cell was tightly compacted to ensure there were no bubbles. Current-time curves and EIS were collected on CHI660E electrochemical workstation CHI660E. Galvanostatic charge/discharge, rate, and cycling measurements were performed on the Neware battery-testing instrument.
The linear-sweep voltammograms (LSV) were recorded with a potential ranging from - 1.15 to -0.75 V (vs. Ag/AgCl) at a scan rate of 2 mV s -1 .

Theoretical calculations
The geometry optimization and adsorption energy of N/O atoms are obtained based on firstprinciples plane wave calculations within density functional theory as implemented in the Vienna ab-initio simulation package (VASP). [3][4][5] The projector augmented-wave method [6] and Perdew-Burke-Ernzerhof exchange-correlation functional [7] are used. A cutoff energy of 400 eV for the plane-wave basis set and a Monkhorst-Pack mesh [8] of 4×4×1 for the Brillouin zone integration are employed for Zn (002), Zn (100) and Zn (101) slabs relaxation and selfconsistent calculations. Zn (002), Zn (100) and Zn (101) slabs with periodically repeating (4×4) unit cell by 5 layers was constructed for the N/O atom adsorption. The lower two layers of atoms are fixed to maintain the same with bulk structure. The thickness of vacuum layer was set to 17 Å. For H 2 O adsorption, the Monkhorst-Pack mesh of 5×1×1 for the Brillouin zone integration is employed for N-graphene ribbon relaxation and self-consistent calculations. All the structures are fully relaxed by conjugate gradient method until the maximum Hellmann-Feynman force acting on each atom is less than 0.01 eV/Å. In our calculation, the Grimme's D3 dispersion correction method is used [9] . The adsorption energy was calculated by the formula: E ads = E total -E slab -E zn , where E total , E slab and E Zn are the total energy with/without the adsorption of Zn atom and atomic energy of Zn, respectively.

Electric field simulation
Finite element analysis conducted by COMSOL Multiphysics software was adapted to compare the electric field distribution at the interface between Zn hexagonal plates on the electrode and electrolyte. According to the two distinguished states of Zn hexagonal plates in the SEM images, two simplified three-dimensional models were established. In the models, one cuboid (8 μm length × 8 μm width × 7 μm height) represents the 2 M ZnSO 4 electrolyte with conductivity of 5 S m -1 , and the other cuboid (8 μm length × 8 μm width × 1 μm height) below the electrolyte represents Zn electrode. Regular hexagon sheets with different sizes were used to present the Zn hexagonal sheets produced during the charge and discharge process. The bigger one, with a base edge of 1 μm and a thickness of 0.25 μm, was twice as the size of the small one. By changing the position state of the Zn sheets, the electric field distribution in different Zn sheet positions and stacking states can be further studied. The electrical conductivity of Zn electrode and Zn sheets was set as 1.   flakes by a modified Hummer's method. [10] To obtain rGO@Zn, bare Zn foil was first soaked in the GO solution for self-assembly and then dried thoroughly in an oven. [11] Because of weak adhesion, the vast majority of rGO falls off from the Zn foil.                            The gray, blue, and red balls denote the zinc, nitrogen and oxygen atoms, respectively.         Supporting Table   Table S1.   Supporting Video Video S1. In situ optical microscopy recording for bare Zn electrode during Zn plating process at 5 mA cm -2 (240 times faster).
Video S2. In situ optical microscopy recording for NOC@Zn electrode during Zn plating process at 5 mA cm -2 (240 times faster).